The present invention relates to methods for depositing crystalline silicon on substrates, and more particularly, it relates to the deposition of undoped or N- or P-doped crystalline silicon in thin films on graphite substrates.
In the electronic industry, there is an increasing need to fabricate devices such as transistors, photovoltaic cells, and microcircuits in which crystalline silicon is deposited in the form of a thin film on the surface of a graphite body, generally denominated a "substrate". Although it has been possible to deposit silicon in thicknesses on the order of 300 microns, up to the present time, there has been no satisfactory method for the deposition of crystalline silicon on graphite in the form of a thin film. Moreover, the prior art deposition methods do not produce a uniform layer.
It is known to deposit certain materials, such as silicon, in the vapor phase on various substrates, such as, for instance, sapphire, spinel, and graphite. These methods for vapor phase deposition do not however permit the production of very thin films, that is, film thicknesses on the order of one to ten microns. Another method, called the diffusion method, involves depositing the crystallizable material on the substrate surface, whereon it then crystallizes. This method also leads to very thin films having thicknesses on the order of one micron. Moreover, in the case of crystalline substances such as silicon, one such method results in a systematic chemical attack on all the substrates.
A recent LEP process, described in the article "Growth of Polycrystalline Silicon Layers on Carbon Substrates for Application as Solar Cells", at pages 191 to 198 of the proceedings of the Colloquium concerning Solar Energy held from Mar. 1 to 5, 1976, at Toulouse, France, relates to the deposition of silicon on a graphite substrate. This method involves immersing the graphite substrate in a silicon bath and pulling the substrate at a selected angle with the horizontal. It is difficult with such a system to obtain a consistent deposition thickness. It is equally difficult to carry out such a method continuously. Moreover, the process requires the maintenance of a considerable quantity of molten silicon in the crucible, and this is very troublesome.
Beyond the foregoing difficulties, none of the prior art methods permits the side-by-side deposition of layers of different substances or different compositions of the same substances. None of these methods, moreover, permits the deposition of a film having a varying composition, continuously or according to a given scheme. It is moreover very difficult, if not impossible, with the methods of the prior art to effect a multiple layer deposit, that is to say, a deposit containing a plurality of layers one on top of the other. Thus, it can be desirable to form deposited films on a graphite substrate, the film comprising bands of silicon alternately doped P and N, or comprising multiple layer films of alternating doped silicon and N-doped silicon, or comprising crystalline silicon films with a continuously variable doping.